Effective Half Life Calculator
Published: May 06, 2026
Source: Ionactive Radiation Protection Resource
Introduction to the effective half life calculator
Effective half-life is used where a radioactive material is being removed by both physical radioactive decay and biological (or other) processes. This calculator combines the physical half-life of a selected radionuclide with a user-entered biological half-life to estimate the effective half-life.
Biological half-life is not a fixed property of the radionuclide alone. It depends on chemical form, route of intake, organ or tissue considered, and the biokinetic model being applied. For this reason, the biological half-life is entered manually by the user. In a later version of this calculator we may provide some biological half-life presets, but for now we provide some examples of use in the notes which follow the calculator.
For a fuller mathematical explanation of physical, biological and effective half-life, see our related Ionactive article here: Physical, biological and effective half-life. If you are looking to perform standard radioactive decay calculations then you may need to look at this Ionactive resource: Radioactive Decay Calculator with Half-Life Analysis & Plotting.
Effective Half-Life Calculator
Biological T1/2 is not a fixed radionuclide property. It depends on chemical form, intake route, organ or tissue considered etc.
Formal advice
If you are after formal advice on calculating effective half life or selecting a biological half life, then head over to our Radiation Protection Adviser (RPA) services , or try our online radiation protection training courses for in-depth study of radioactive decay and half life.
Overview of effective half life
Physical half-life describes how quickly a radionuclide decays by radioactive decay alone. Biological half-life describes how quickly material is removed from the body, organ or compartment being considered by biological processes such as excretion or clearance. Although not explored in this calculator, radioactive material removal could also be considered in the following circumstances:
- Ventilation / air exchange half-time e.g. where ventilation is used to remove short half life radioactive material (e.g. F-18).
- Chemical separation / ion exchange / purification half-time where radioactive material is taken out of a system whilst it is also decaying.
- Radionuclide generator elution systems (e.g. Tc-99m generator). This is beyond our simple calculator, but the same principles are used.
Where both processes occur at the same time, the effective half-life describes the combined rate of removal. The effective half-life is always shorter than either the physical half-life or the biological half-life alone, because both removal mechanisms are acting together. We can write Teff in the following form:
\( \frac{1}{T_{\mathrm{eff}}} = \frac{1}{T_{\mathrm{phys}}} + \frac{1}{T_{\mathrm{bio}}} \)
Then by rearrangement we have:
\[
T_{\mathrm{eff}} = \frac{T_{\mathrm{phys}} \times T_{\mathrm{bio}}}{T_{\mathrm{phys}} + T_{\mathrm{bio}}}
\] Where,
\( T_{\mathrm{phys}} \) = physical half-life
\( T_{\mathrm{bio}} \) = biological half-life
\( T_{\mathrm{eff}} \) = effective half-life
As noted earlier, the full maths and derivation is available here: Physical, biological and effective half-life.
Example uses of the effective half life calculator
All the examples that follow are illustrative only - there are many variables and individual circumstances which could change considerably the values and data given in this section.
Po-210
Physical half life of 138 days, and a biological half life of about 40 days. This produces the following plot in our calculator.
Po-210 effective half life calculator example
H-3
Physical half life of 12.3 years, and a biological half life of about 10 days (depends on form of H-3). This produces the following plot in our calculator.
H-3 effective half life calculator example
Cs-137
Physical half life of 30 years, and a biological half life of about 110 days. This produces the following plot in our calculator.
Cs-137 effective half life calculator example